The present invention relates to a dual stroke cylinder in which a piston rod can be stopped in an intermediate position of the stroke.
A normal hydrostatic pressure cylinder is so constructed that a piston rod thereof moves from the starting end to the termination of the stroke all at once. However, there is a demand according to the operation contents for stopping the piston rod in an intermediate position of the stroke, performing some operations during the stroke up to that position, and moving the piston rod again up to the termination of the stroke to perform an operation for the next stage. Moreover, energization of a solenoid valve for controlling the hydrostatic pressure may be cut off due to an unforeseen situation. In that case, with a normal hydrostatic pressure cylinder, a part of an operator""s body may be put between work pieces mounted on the rod, at the starting end or termination of the stroke of the piston rod. As a safety measure for preventing such a problem, it is more effective to use a hydrostatic pressure cylinder which can be stopped in an intermediate position and designate the intermediate stop position as a non-energized origin, than a method using a lock mechanism or a three position valve.
FIG. 6 shows one example of a dual stroke cylinder already proposed by the present applicant, in which a piston rod can be stopped in an intermediate position of the stroke. This dual stroke cylinder 1 comprises: a cylinder tube 2A, a first piston 3A sliding airtightly in the cylinder tube 2A; an annular second piston 3B sliding airtightly in the cylinder tube 2A on the rod side of the first piston 3A; a hollow piston rod 7 integral with the second piston 3B; a head cover 2B; and a rod cover 2C.
Inside the cylinder tube 2A, there is provided a stopper rod 6 for setting the stroke of the first piston 3A, which penetrates the center of the first piston 3A so that it can slide freely and airtightly, with one end thereof being fitted to the head cover 2B.
Moreover, the hydrostatic pressure cylinder 1 comprises: a first port 5A for supplying a pressure fluid to a first cylinder chamber 4A on the head cover 2B side of the first piston 3A; a second port 5B for supplying a pressure fluid to a second cylinder chamber 4B between the first piston and the second piston via the inside of the stopper rod 6; and a third port 5C for supplying a pressure fluid to a third cylinder chamber 4C on the rod cover 2C side of the second piston 3B.
Therefore, when the pressure fluid is supplied from the first port 5A to the first cylinder chamber 4A, the first piston 3A is driven, and at the same time, the second piston 3B and the piston rod 7 pushed thereby are driven in the same direction. When the first piston 3A slides as far as the stopper 6B located at the termination of the stopper rod 6, the first piston 3A stops at a position which is moved to the left by a stroke S1 in the figure, and then the pushed second piston 3B and rod 7 stop in the intermediate position of the stroke.
In this state, when the pressure fluid is supplied from the second port 5B to the second cylinder chamber 4B between the first piston and the second piston, and if the pressure fluid has been introduced into the third cylinder chamber 4C, when the pressure fluid is discharged according to need, the second piston 3B and the rod 7 thereof which are stopped in the intermediate position of the stroke move to the left by the remaining stroke (S2-S1) and stop at the termination position of the driving stroke.
This dual stroke cylinder 1 can stop the rod 7 of the second piston 3B to the intermediate position of the stroke S2. However, since the first piston 3A bumps against the stopper 6B of the stopper rod 6 at the time of stopping in the intermediate position, the stopper rod 6 undergoes an impact force every time the first piston 3A is driven.
Accordingly, it is necessary to give strength to the stopper rod 6 so as to endure the impact force. When the strength of the stopper rod 6 is increased, the diameter of the stopper rod 6 increases, and the inner diameter of the first piston 3A which slides on the outer circumference of the stopper rod 6 also increases. Hence, the area presented to the wind of the first piston 3A decreases by that amount, causing a problem in that the output of the first piston 3A decreases.
Furthermore, if the stopper rod 6 is made large, the rigidity of the stopper rod 6 increases, and hence the stopper rod 6 cannot be bent, and as a result, it becomes necessary to strictly adjust the parallelism of the stopper rod 6 and the cylinder tube 2A.
It is an object of the present invention to solve the problems in the dual stroke cylinder already proposed and to provide a dual stroke cylinder in which a stopper rod is not required.
To achieve the above object, the dual stroke cylinder of the present invention is characterized in that it has: a cylinder tube having a head cover and a rod cover fitted to the opposite ends thereof; first and second two pistons which slide airtightly in the cylinder tube; cylinder chambers respectively partitioned between the first piston and the head cover, between the two pistons, and between the second piston and the rod cover; ports for supplying a pressure fluid respectively to each cylinder chamber; a piston rod with the base end portion being connected to the second piston, and the top end portion penetrating the rod cover airtightly, and extending to the outside of the cylinder tube; a hollow rod for intermediate stop having a stopper at the top end portion, with which the second piston is engaged and stops, with the base end portion being connected to the first piston, and with the top end portion penetrating the second piston so as to be relatively freely movable in the back and forth direction, and going into the inside of the piston rod; a conduit shaft having a fluid passage connecting a hollow portion in the piston rod and the second port inside thereof, with the base end portion being connected to the head cover and airtightly and slidably penetrating the first piston and the rod for intermediate stop, and with the top end portion being located in the hollow portion of the piston rod.
In the present invention, it is preferable to have position adjusting means that can adjust the position of the first piston at a return end along the axis of the piston. This position adjusting means is desirably a bolt screwed into the head cover so as to freely advance and retreat, with the top end protruding into the first cylinder chamber.
In the present invention, the stopper fitted to the top end of said rod for intermediate stop is position-adjustable in the axial direction of the rod for intermediate stop.
In the dual stroke cylinder having the above-described construction, the second piston is slid by means of s pressure fluid supplied from the second port to the second cylinder, and the second piston can be stopped at an intermediate position of the stroke by means of the stopper and the second piston and the rod thereof can be held at that intermediate position. By supplying the pressure fluid from the first port to the first cylinder in that state, the first and second pistons and the piston rod can be moved to the stroke termination position.